The present invention relates to an electric disk brake operable to generate a brake force by pressing a brake pad against a disk rotor with use of an electric motor, and in particular, to an electric disk brake comprising a parking brake mechanism capable of holding a brake force while an electric current is not applied.
There is known an electric disk brake which generates a brake force by converting a rotation of a rotor of an electric motor into a linear motion of a piston through a rotation-linear motion converting mechanism, such as a ball screw mechanism or a ball ramp mechanism, and pressing a brake pad against a disk rotor by the piston. Such an electric disk brake can generate a desired brake force by detecting a force that a driver applies to a brake pedal (or a displacement amount of the brake pedal) with use of a sensor, and controlling a rotation of the electric motor based on the detection value with use of a control apparatus.
Some electric disk brakes of this kind comprise a parking brake mechanism capable of maintaining a braked state even after the application of an electric current to the electric motor is stopped by mechanically locking a rotation of the rotor of the electric motor, as disclosed in, for example, Japanese Patent Application Public Disclosure No. 2005-9633. The parking brake mechanism is configured as follows, so as to be able to maintain a braked state even when an electric current is not applied; that is, a ratchet wheel is attached to the rotor of the electric motor, and after the rotor of the electric motor is rotated to be situated at a brake position, the ratchet wheel is locked at that position by the engagement of an engagement pawl with a tooth of the ratchet wheel with use of a solenoid actuator.
As another example, Japanese Patent Application Public Disclosure No. 2007-203821 discloses an electric disk brake comprising a parking brake mechanism and being capable of determining an abnormality or a failure of a mechanical lock mechanism of the parking brake mechanism when the brake is not in operation.
In the above-mentioned electric disk brakes comprising the parking brake mechanism, the parking brake mechanism may be actuated while the brake pad and the disk rotor have a high temperature immediately after the vehicle is braked. In this case, the brake pad and the disk rotor thermally-expanded under the high temperature may be contracted due to a reduction in the temperature after a certain time has passed, resulting in a reduced brake force. Therefore, the need arises for preventing the reduction in the brake force by automatically re-actuating the parking brake mechanism after a certain time has passed while the vehicle is parked.
However, a problem may be derived from this re-actuation of the parking brake mechanism; if a vehicle is in such a state that the mechanical lock does not work properly due to a malfunction, once the mechanical lock is released for the purpose of automatic re-actuation of the parking brake mechanism while the vehicle is parked, the relock thereof is impossible, whereby the actuated state of the parking brake cannot be maintained. On the other hand, in the electric disk brake disclosed in Japanese Patent Application Public Disclosure No. 2007-203821, the abnormality determination can not be performed while the vehicle is parked since it is performed while the brake is not in operation.